From the standpoint of recent environmental protection in terms of global warming prevention, a fuel cell generating no carbon dioxide gas is attracting attention. A hydrogen fuel cell is representative of this fuel cell. However, a hydrogen fuel cell is disadvantageous in that a combustible gas such as hydrogen gas, natural gas, etc., must be used as the fuel (proton source). On the other hand, a fuel cell includes a biofuel cell using biomass as the proton source. A biofuel cell has the advantage of not using a combustible gas. In addition, although a biofuel cell results in carbon dioxide emissions, as a result of using a plant-derived biomass, it does not increase net carbon dioxide. Furthermore, a biofuel cell is being taken notice at present in light of recycling of organic resources, and various kinds of biofuel cells have been proposed.
One example thereof is PTL 1.
In the biofuel cell of PTL 1, a negative electrode chamber (fuel electrode chamber) provided with a negative electrode and a positive electrode chamber (oxygen electrode chamber) provided with a positive electrode are partitioned by a proton permeable membrane (diaphragm). In the negative electrode chamber, an electron mediator is present together with a biocatalyst such as enzyme capable of metabolically degrading the biomass. On the other hand, the positive electrode chamber is filled with an electrolytic solution containing, for example, a polyvalent metal ion (e.g., Fe3+), and oxygen is blown thereinto.
In such a biofuel cell, an electron is produced in the negative electrode chamber due to biomass degradation by the biocatalyst. This electron is transferred to the negative electrode by the electron mediator. The electron reached the negative electrode works in an external circuit and then reaches the positive electrode. The electron reached the positive electrode reduces a high-valence polyvalent metal ion in the positive electrode chamber and produces a low-valence polyvalent metal ion (Fe3+→Fe2+). Also, a proton is produced in the negative electrode chamber due to biomass degradation by the biocatalyst. This proton is introduced into the positive electrode chamber through the proton permeable membrane and on the positive electrode, is oxidized by oxygen together with the low-valence metal ion. As a result, water is produced and at the same time, the low-valence metal ion returns to the original high-valence metal ion. A series of these reactions are represented, for example, by the following formulae:
<Negative Electrode Chamber>C6H12O6+6H2O→6CO2+24H++24e−
<Positive Electrode Chamber>e−+Fe3+→Fe2+4H++4Fe2++O2→2H2O+4Fe3+
Such electrode reactions are continuously performed, whereby an electric current is produced to generate electricity.